The present inventors examined a method of forming one bipolar transistor of a large current by connecting a plurality of small bipolar transistors in series for a semiconductor device. According to this method, many kinds of large-current bipolar transistors can be formed easily while maintaining high performance of the bipolar transistors. The plurality of small bipolar transistors are disposed on a semiconductor layer to form an element that is provided on an insulation layer of the SOI substrate. Each of the small bipolar transistors is surrounded with an isolation.
Japanese Patent Application Laid-open Publication No. 10-173040 discloses the following configuration, for example. A U-trench isolation surrounds each of bipolar transistors having a common collector. Each silicon island surrounded with the U-trench isolation is comprised of a collector of each bipolar transistor, thereby to make it possible to change a collector potential.
Japanese Patent Application Laid-open Publication No. 2002-57219 discloses a configuration having a separator formed in an isolation region that isolates a semiconductor element in a saturation state from a semiconductor element that is not in a saturation state, thereby to avoid mutual move of charges.